Miniature mass spectrometers have application as portable devices for the detection of biological and chemical warfare agents, drugs, explosives and pollutants, as instruments for space exploration, and as residual gas analysers. Further applications exist for low cost systems in pharmaceutical analysis. Mass spectrometers consist of three main subsystems: an ion source, an ion filter, and an ion counter.
One of the most successful variants is the quadrupole mass spectrometer, which uses an electrostatic quadrupole as a mass-filter [Paul 1953]. Conventional quadrupoles consist of four cylindrical electrodes, which are mounted parallel and with their centre-to-centre spacing at a well-defined ratio to their diameter [Dawson 1976; Denison 1971]. Ions are injected into the pupil between the electrodes, and travel parallel to the electrodes under the influence of a time-varying field approximating an ideal hyperbolic potential variation. This field contains both a direct current (DC) and an alternating current (AC) component. The frequency of the AC component is fixed, and the ratio of the DC voltage to the AC voltage is also fixed. Studies of the dynamics of an ion in such a field have shown that only those ions with a particular charge to mass ratio will transit the quadrupole without discharging against one of the rods. Consequently, the device acts as a mass filter. The ions that successfully exit the filter may be detected. If the DC and AC voltages are ramped together, the detected signal is a spectrum of the different masses that are present in the ion flux. The largest mass that can be detected is determined from the largest voltage that can be applied.
If the DC component is omitted, and the quadrupole is operated in RF-only mode, the action of the field is different, and the quadrupole acts as an all-pass filter or ion guide [Dawson 1985; Miller 1986]. When ion transmission is interrupted by collisions with other ions or neutrals, a form of ion focusing known as collision focusing takes place. As a result quadrupole ion guides have a variety of applications in mass spectrometers, including devices for enhancement of transmission and devices for ion fragmentation known as collision cells [Douglas 1998].
An electrostatic quadrupole may also be used in an alternative form of mass spectrometer known as a linear quadrupole ion trap [Prestage 1989]. If suitable barrier potentials are provided using additional electrodes at the ends of the quadrupole, ions may be confined inside the quadrupole and perform multiple transits up and down its axis. Ions may be mass selectively ejected from the exit by a variety of means including using an auxiliary AC voltage applied to the end electrode, at the same time as ions are admitted at the entrance [Hager 1989; U.S. Pat. No. 6,177,668]. Improved filter performance follows from the increased number of RF cycles experienced by the ions and increased signal-to-noise ratio follows from the accumulation of ions inside the trap. A linear ion trap may also be operated using time gating of the end potentials [Campbell 1998].
The resolution quadrupole filter is determined by two main factors: the number of cycles of alternating voltage experienced by each ion, and the accuracy with which the desired field is created. So that each ion experiences a large enough number of cycles, the ions are injected with a small axial velocity, and a radio frequency AC component is used.
The accuracy with which the field is created is determined by the shape and size of the electrodes, and by their placement and their straightness. Numerous studies have shown that a good approximation to a hyperbolic field is provided by cylindrical electrodes with their centre-to-centre spacing at a well-defined ratio to their diameter. However, a reduction in the mass resolution is caused by misplacement of the electrodes [Dawson 1979], by bending of the electrodes [Dawson 1988] or by other distortions of the field.
To avoid such problems, highly accurate methods of construction are employed. However, it becomes increasingly difficult to obtain the required precision as the size of the structure is reduced. Microfabrication methods are therefore increasingly being used to miniaturise mass spectrometers, both to reduce costs and allow portability. These processes are generally carried out on planar substrates, which are often silicon or multilayers containing silicon. The most important of the processes considered here include:                Patterning methods such as photolithography;        Etching methods such as deep reactive ion etching of silicon;        Bonding methods such as anodic bonding of silicon and direct bonding of silicon;        Isolation methods such as oxidation of silicon;        Coating methods such as sputtering of metals;        Interconnection methods such as thermocompression bonding of gold wire.        
These methods are well known to those skilled in the art, and can be employed in many different combinations to achieve a given microstructured object.
For example, a miniature silicon-based quadrupole electrostatic mass filter consisting of four cylindrical electrodes mounted in pairs on two oxidised silicon substrates was demonstrated some years ago. The substrates were held apart by two cylindrical insulating spacers, and V-shaped grooves formed by anisotropic wet chemical etching were used to locate the electrodes and the spacers. The electrodes were metal-coated glass rods soldered to metal films deposited in the grooves [U.S. Pat. No. 6,025,591].
Mass filtering was demonstrated using devices with electrodes of 0.5 mm diameter and 30 mm length [Syms et al. 1996; Syms et al. 1998; Taylor et al. 1999]. Performance was limited by RF heating, caused by electrical coupling between co-planar cylindrical electrodes through the oxide interlayer via the substrate. As a result, the device presented a poor electrical load, the solder attaching the cylindrical electrodes tended to melt, and differences in expansion coefficient between the cylindrical electrodes and the substrate caused the electrodes to detach. These effects restricted the voltage and frequency that could be used, which in turn limited the mass range (to around 100 atomic mass units) and the resolution.
In an effort to overcome these limitations, an alternative miniature construction based on bonded silicon-on-insulator (BSOI) was developed [GB 2391694]. BSOI consists of an oxidised silicon wafer, to which a second silicon wafer has been bonded. The second wafer may be polished back to the desired thickness, to leave a silicon-oxide-silicon multi-layer. In this geometry, cylindrical stainless steel electrodes were mounted in pairs on two substrates. The oxide interlayer was largely removed, so that electrical coupling between co-planar cylindrical electrodes via the substrate was greatly reduced. As a result, the device could withstand considerably higher voltages, and a mass range of 400 atomic mass units was demonstrated [Geear 2005].
The cylindrical electrodes were retained by the pressure contact of two silicon springs etched into the substrate of the BSOI wafer. The spring retaining system allowed a sliding motion of the electrodes, so that variations in temperature did not cause strains due to differences in the thermal expansion coefficients of the stainless steel cylindrical electrodes and the silicon mount. However, the sliding motion allowed the position of the electrodes to alter slightly, either over long periods of time or following mechanical shocks, degrading the constructional accuracy of the filter.
There is therefore a benefit in rigidly retaining the electrodes on the supporting substrates but there are still problems regarding differences in thermal expansion coefficients between the electrodes and their respective mounts.